Understanding AC to DC Transformers in Electronics Design

Key Takeaways

• Understand what an AC to DC transformer is

• Learn how to design an AC to DC transformer

• Explore the limitations of AC to DC transformers

I recently introduced my son to the world of “Transformers.” It’s fascinating how the franchise of vehicles-turned-robots has not only survived but thrived for decades. Naturally, he quickly became a fan of Optimus Prime, and it wasn’t long before I was persuaded to order an exact toy replica.

It seems that my son’s obsession with Transformers is going to cost me a few more hundred dollars in the near future. In my line of work, the knowledge of building another type of transformer—the AC to DC transformer—can save me some money in buying ready-made ones. 

What Is an AC to DC Transformer?

A typical AC to DC transformer.

A transformer in electronics design is no shape-shifting robot. It is a component that consists of a common iron core with at least two wire windings around it. The transformer is used to step down or step up the AC voltage, with the principle of electromagnetic induction.

When you apply AC voltage to the primary winding, energy is stored in the core and coupled over to the secondary winding. Depending on the turns ratio, the secondary winding will produce an AC voltage accordingly. A step-down transformer will have a higher number of primary windings and the opposite is true for a step-up transformer.

The term AC to DC transformer refers to a transformer that is connected to an AC rectification circuit. After increasing or decreasing the AC voltage, the rectification circuit converts the AC voltage to DC voltage. 

An AC to DC transformer is a simple solution for powering up electronics from the AC mains. Often, you’ll find AC to DC transformers in the form of an adapter that plugs into the mains socket. 

AC to DC Transformer Design

To cobble up an AC to DC transformer, you’ll first need to choose a transformer with the correct winding ratio. You can use the formula of:

Primary voltage / Secondary voltage = Primary Turns / Secondary Turns

You’ll also need to consider the material and size of the transformer, as they could affect its current load rating. Choose a transformer that could reliably convert the energy for the load in the circuitry.

The key to designing an AC to DC transformer lies in its rectification circuit. You’ll have two types of circuits to choose from—half-wave or full-wave rectification.

Half-wave rectification involves connecting a single diode in series with the secondary winding. The result is only the positive cycle of the AC voltage is allowed to pass through. 

Output of a half-wave rectifier where only the positive cycle of the AC voltage can pass through.

A capacitor with a large value is placed across the secondary output to even out the waveform to produce the desired DC output. However, half-wave rectification isn’t efficient—it’s difficult to smooth such an uneven output.

Full-wave rectification is a better alternative to convert the AC voltage into DC. This method involves connecting the secondary AC output to a full-wave diode-bridge rectifier. Instead of merely chopping the negative cycles, the full-wave rectifier flips the negative cycle into positive ones. 

The output of a full-wave rectifier, a better alternative to converting AC voltage into DC because it flips the negative cycles into positive ones.

Obviously, opting for full-wave rectification is the better option. At the expense of four diodes, you’ll have a less-choppy DC output, which means a lower value capacitor can be used to smooth the peaks. Full-wave rectification is also more efficient, as the energy from negative cycles is converted and transferred to the load. 

Limitations of AC to DC Transformers

Despite the simplicity of AC to DC transformers, there are limitations. To start, the DC voltage output is susceptible to fluctuation on the primary input. Therefore, it’s never a good idea to connect a microcontroller or ICs directly to an AC to DC transformer. 

AC to DC transformers are also inefficient as most of the energy is dissipated as heat. Transformers are also costly and take up too much space in the design. Still, AC to DC transformers can be an economical generic solution when manufactured in bulk. 

Designing an AC to DC transformer is made easier with the right PCB design software. OrCAD PCB designer has all the necessary footprint and layout capability to get one finished in little time. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.